Exposure control



Dec. 14, 1937.

J. EGGERT AL 2,101,932

EXPOSURE CONTROL Filed April 19, 1935 2 Sheets-Sheet 1 In ven fors:

By Afforneys Dec. 14, 1937. J. EVGGERT AL 2,101,932

EXPOSURE CONTROL Filed April 19, 1935 2 Sheets-Sheet 2 Afforneys m, W

- ilnvenfo rs:

Patented Dec. 14, 193'] um'rso STATES PATENT OFFICE I r f 2,101,932 vJohn Eue Leipzig-Gohlis, and Hans Friedrich u, Germany, assignors to I.G.

Farbenindustrie Aktlengesellschaft, Frankforton-the-Main, GermanyApplication April 19, 1935, Serial No. 17,318 In Germany April 20, 1934lOlaim.

Our present invention relates to exposure control and more particularlyto an apparatus for the application 01' a dosed exposure in thephotographic reversal process and in the photographic printing process.

One of its objects is to provide an improved apparatus for exposurecontrol. Another object is an apparatus of this kind which is moreefiicient than those hitherto known. A further object is an apparatusfor exposure control which is operated exclusively optically andelectrically and is therefore operated with a minimum of in ertness.Further objects will be seen from the detailed specification followinghereafter. Reference is made to the accompanying drawings in which:

Fig. 1 shows a diagram of the connections of an apparatus according tothis invention,

Fig. 2 shows a diagram of the connections of an apparatus according tothis invention as shown in Fig. 1 with the diiference that a specialrelay has been provided for holding the required contacts closed whenthe galvanometer has a too great deflection,

Fig. 3 shows a diagram of the connections of an apparatus according tothis invention in which the intensity of the printing light is regulatedover a heavy current valve,

Fig. 4 shows a modified apparatus according to this invention,

Fig. 5 shows an apparatus as shown in Fig. 4 but provided with a backcoupling,

Fig. 6 shows an apparatus as shown in Fig. 5 with a modified backcoupling, and

Fig. 7 shows a diaphragm as usedin the apparatus shown in Figs. 4 to 6.

In the second exposure in the production of a reversed film it isnecessary to adjust the quantity of light projected on the film inaccordance with the density of the photographic image. So also in theprinting process the illumination must be selected in dependence on thedensity of the negative.

A known arrangement for producing an automatic adjustment of the lightconsists in placing on the other side of the film to be illuminated adevice whereby the transmitted light excites a photo-electric cell thecurrent from which moves directly or indirectly a diaphragm in the pathof the rays from the illuminating lamp. These devices have the drawbackthat they are comparatively slow because considerable masses have to bemoved, with the disadvantage that in the majority of cases the film or agreater part of it is incorrectly exposed.

According to this invention the exposure of the film is regulatedexclusively electrically and optically. With the apparatus hereindescribed it is not only possible to control the intensity of theprinting light in any desired proportion, but it is also possible toadjust the intensity of the printing light according to the density ofthe film to be exposed while having regard to the specialproperties ofthe used film and the used photocell. A particular advantage of theapparatus according to this invention is its little inertness whichguarantees a rapid adjustment of the intensity. Photo-electric cells ofthe regenerative type that is to say such which are capable to convertlight into electric energy are preferred, for instance, photo-electriccells in which a layer of a finely subdivided semi-conductor is incontact with a layer of a conductor, as in the cuprous oxide cells.However, also the other photo-electric cells may be used. The use ofnon-regenerative photo-electric cells that is to say such in which onlythe resistance to an electric current is changed by the influence oflight are less practical because they need a special source of electriccurrent and an amplifier.

In using the apparatus for dosing the intensity of the source of lightin the second exposure of a reversal film it is not necessary to scanthe whole breadth of the pictures. The desired width which must,however, not include the perforation row, is blocked out by a mask. Inorder to obtain correct density of the print at least one picture lengthmust be scanned, preferably there are scanned 4 to 5 picture lengths.

The modifications of an apparatus for exposure control according to thisinvention have the common feature that the light projected, forinstance, through the bleached reversal film excites a photo-electriccell. The current of the photoelectric cell actuates a galvanometerprovided with a mirror. According to the variations of power of thecurrent the galvanometer is more or less deflected and, therefore, thebeam of light projected from a source of light on the mirror. This beamof light may be used directly or indirectly for exposing the film. Forusing it directly the beam of light falls on a fixed mask having asuitable aperture which according to the degree of deflection transmitsmore or less light which is evenly distributed on the film plane to beexposed. When using the beam of light indirectly it falls onphoto-electric cells which by means of relays switches-in or cuts outresist-' ance in the circuit of the source of light.

In printing from a finished negative or posi- The principle of theindirect method will now be explained with reference to the accompanyingdrawings. The indicesof the switches are the same as those of relays bywhich they are actuated. The reversal film F which is to be exposed ispassed between a source of light and a photo-electric device in such amanner that the photo-electric cell is excited by the light passingthrough the film. The current produced by the light in thephoto-electric cell actuates a galvanometer G provided with a mirror.The mirror reflects a light ray of the source "of light Lg- The positionof the mirror determines the direction of the reflected light ray. Thusthe direction of the light ray is dependent on the density of thescanned picture. .With a certain deflection of the mirror of thegalvanometer the light is projected on a'photo-electric device B1, whichactuates a relay C1. The relay initiates the adjustment of the requiredintensity of the source of light which serves for the second exposure ofthe film. There are arranged several photoelectric devices B1, B2 B11directly side by side or at suitable distances, each photo-electricdevice actuating a special relay. In accordance with the relay which isactuated a oertain intensity of the source of light for exposing thefilm is produced.

The arrangement represented in Fig. 2 shows one of the manypossibilities. If the relay C1 is actuated by light projected on B1, asecond relay D1 is energized so that it closes the switch df, Hereby theresistance R1 is short-circuited so that the current supplied to thesource of light L11 is increased and the intensity of the source oflight likewise is increased. If the relay C2 is actuated R2 isshort'circuited over czDzdg. The intensity of the source of light 1111is further increased. Thus, the intensity of the source of light may beregulated to difierent values which are determined by the value of theresistance R.

In the following manner the efiectiveness may be further increased. Thepencil of light projected on the photo-electric devices B1, B2 and so onis made so large that when the pencil of light passes from onephoto-electric device to the next two adjacent relays are actuated(apart from the first and the last photo-electric device). By closingthe switch d5 with the actuation of the relay D1 the relay is kept inaction also when 01 is not actuated by C1. This involves the advantagethat when the pencil of light emanating from the mirrorin moving backfrom E2 to E1 the switch 01 is closed without being under tension.Furthermore d; is closed, so that on actuation of d? only the partialresistance R2 is switched on or switched ofl. This would not be the caseif, for instance, the galvanometer G is operated so quickly that therelay cannot follow immediately. This involves a steady operation,little wear, and increased reliability in service. It is not desiredthat all relays are in operation; the switches d which are actuated bythe relays D having the-corresponding indices switch off all relaysexcept two which are in operation.

' If the galvanometer G has no abutment which prevents deflection of thepencil of light so that it passes beyond .1311 it is possible to providea devicex which retains the relay D11 in its operative position when thepencil of light is deflectedbe-- yond B11. For this purpose that relayhas a coil D11 which is actuated over a switchdfi of the relay D11 andan open circuit contact c11'-1'of the relay C'11 1. If the pencil oflight wanders from B11 to EH, 3111-: the relay D11 must be switched ofi.If it wanders in the opposite direction the relay D11 must remain inoperation. Thus, the maximum intensity of the source of light 1111 whichmay be required cannot be surpassed.

Another possibility of control consists in applying difierent gridvoltages to the grid of a heavy-current control valve by means of therelays C1 C11. The switching onof diilerent voltages may be obtained indifierent manners, for instance, by means ot a potentiometer, a batteryfrom which different voltages can be taken, a transformer from whichdifferent voltages can be taken or an apparatus by which a phase lead ora phase lag is produced. Thealternating current which flows through theheavycurrent control valve is directly or after transformation conductedto the lamp which serves for the exposure of the sensitive film.

The direct method provides a rapid adjustment by using a stationarydiaphragm and changing the direction of the light rays so that more orless light is projected upon the diaphragm.

The accompanying diagrams of Figs. 4, 5, and 6, illustrate threearrangements in accordance with the invention. 1

Referring to Fig. 4, the film F is illuminated by the source of lightL1. The transmitted light excites the photo-electric cell A, the currentfrom which actuates a mirror galvanometer G.,

This mirror reflects the cone of rays from the lamp L2. In the path ofthis reflected light is the diaphragm B, the form of which is preferablysuch that a certain ratio between the density curve of the image and thegradation of the 'strength of illumination is obtained. The diaformthere may be used a diaphragm with the interposition of a suitable greywedge or there may be used a grey wedge alone. The diaphragm isadvantageously placed at a position in the path of the rays where thediameter of the cross section of the cone of light corresponds withabout the greatest breadth of the diaphragm.

If the current passing through the galvanometer becomes stronger inconsequence of a smaller density of the image, the cone of rays movesgradually behind the diaphragm, the reverse being the case when thedensity of the image becomes greater. The galvanometer mirror may be soadjusted that there is always a certain minimum quantity of lightpassing through the diaphragm.

There can, however, be provided a separate passage for the light.Advantageously, the lamp Li,

which serves for providing the transmitted light,

may have such photographic activity that it pro- 5 duces the smallestnecessary illumination. The light passing through the diaphragm fallsupon the ground glass screen D by which the film is luminated. The lossof light which occurs at the ground glass must be taken into account inselecting the intensity of the source of light. In order to avoid lossof light the path of the light between the ground glass screen and thefilm is surrounded by reflecting surfaces E.

Fig. 5 shows an arrangement for increasing the sensitiveness of thedevice by mea s of an auxiliary current.- n the left hand si e is thelamp L that provides the light rays to be transmitted by the film, andon the right the illuminating lamp vN. In parallel with thephoto-electric cell A, 20 which actuates the galvanometer G, there isinserted a second photo-electric cell H, which is excited by theilluminating lamp N. The light necessary for the actuation of thephoto-electric cell H may be taken from the trace of the rays of the 25printing light N at any place behind the dia phragm B. This may be doneby introducing a suitably inclined mirror into the trace of the rays.

The photo-electric cell H may, as shown in Fig.

be arranged behind the film and a too great in- 30 tensity of the lightpassed through the film may be reduced by the insertion of a. suitablemask. The regulation of the additional energy obtained from the cell Hmay also be achieved by the re sistance RH- For regulating anyfluctuations in the illumination of the lamp L or of the lamp N thefollowing devices are provided:-In series with the photo-electric cellsA and H, which serve for regulating the illumination, are inserted twolike cells J and K with interposition of resistances RJ and Rx; theseare directly illuminated by the lamps L and N. When, for example, thecandle power of the lamp diminishes, the countercurrent from the cell Jor K through the galva nometer diminishes. The device has still anotheradvantage. According to experience the density of a film which is toreceive a second exposure varies very little. Correspondingly the lightfalling on the photo-electric cell A changes only in small degree; soalso does the current from the photo-electric cell which actuates thegalvanometer. One can think of these currents together as a constant andone which changes with the density of the image. This constant currentis a disturbing factor and is advantageously removed by an equal,opposed current in the gelvancmeter. This is derived from thephotoelectric cell J or K. A correct magnitude of the current is securedby a diaphragm N or O or by the resistance R; or RE.

A further possibility for producing the auxiiiary energy is shown inFig. 6. Here the light that is transmitted through the film and theilluminating light are so close to each other that they inpart overlap,whereby the auxiliary energy is produced. The current is adjustable by adiaphragm P placed in front of or behind the film F. For this purposethe diaphragm can be laterally displaced. Advantageously the diaphragmhas reflecting faces. This illuminating device has the advantage that itis very simple and operates very quickly. Obviously instead of a singlesource of light for producing the transmitted rays and for theillumination there may also be used two sources of light.

What we claim is:

An apparatus for exposure control comprising in combination a source oflight for scanning a picture, a photo-electric device which receives thelight varying in intensity with the density of said picture therebycausing a variation of an electric current, a galvanometer deflected inaccordance with said variations of electric current, a mirror providedon said galvanometer, a second source of light projecting a pencil oflight on said mirror, said pencil of light being reflected by saidmirror, a series of photo-electric devices arranged one beside the otherWithin the reach of said reflected pencil of light at a distance smallerthan the breadth of the reflected pencil, relays arranged in a circuitwith said photo-electric devices, a further series of relays actuated bysaid first named relays, a source of light for exposing a photographiclayer, a resistance arranged in a circuit with said source of light andswitches actuated by said relays for switching on and switching offparts of said resistance for varying the intensity of said source oflight for exposing said layer in accordance with said variations ofelectric currrent, a wire coil arranged on the relay situated farthesttowards the end of the reach of said reflected pencil of light when saidmirror is most deflected, and switches actuated by said wire coil forkeeping switched off the whole resistance in the circuit of said sourceof light for exposing said layer when said refiected pencil of lightpasses beyond the last photo-electric device of said series.

JOHN EGGERT. HANS FRIEDRICH NISSEN.

